1. Field of the Invention
The present invention relates to a process of fabricating a semiconductor device and, more particularly, to a process of forming a projected electrode on an Al electrode pad for the connection of the Al electrode to the outside.
2. Description of Related Art
As one of the presently used methods for forming a projected electrode for the connection of an Al electrode pad to the outside, the electroless plating method is pointed out. In case of the electroless plating method, the sputtering step, the photolithography step and the etching step can be omitted, as compared with the electro-plating method, and therefore, the manufacturing costs can be reduced, which attracts attention.
However, in case a surface oxide film exists on the Al electrode pad, it becomes difficult to form a projected electrode with a desired shape. Due to this, the reliability in the connection of the projected electrode lowers. Thus, in case of performing electroless plating, it is important to remove the surface oxide film on the Al electrode pad which is a treatment performed before the plating. If the pre-treatment (a treatment performed with a solution containing Zn) for the electroless plating is performed in the state in which the surface oxide film remains on the Al electrode pad, then the substitute between Al and Zn cannot be performed with uniformity and denseness. Due to this, the core of the Ni or Ni alloy which constitutes the projected electrode and substitutes with Zn when the electroless plating is performed, is coarsely deposited. The Ni or Ni alloy which is coarsely deposited is also low in bond strength. Further, due to coarse deposition, there arises problems such as the problem that corrosion is caused due to the impurities penetrated from the outside, which lowers the reliability.
In connection with such problems, in Japanese Unexamined Patent Publication No. HEI 9(1997)-326395, the method according to which the surface oxide film is dissolved by the use of a solution such as sodium hydroxide, phosphoric acid or the like is pointed out as an example of the removal of the surface oxide film on an Al electrode pad.
Further, in Japanese Unexamined Patent Publication No. HEI 9(1997)-306871, there is disclosed the method according to which the surface oxide film on an Al electrode pad is removed by sputtering, and Ni-plating is carried out directly in an inert atmosphere.
FIGS. 4(a) to (d) are sectional views of the structure at respective fabricating steps in case the surface oxide film on the Al electrode pad is removed, using the solution disclosed in Japanese Unexamined Patent Publication No. HEI 9(1997)326395, and then, a projected electrode is formed. So far, this method has been used, but there is the problem that a dispersion is caused in the etched amount of the Al electrode pad, depending on the useful life and the frequency in use of the solution (etchant). In this case, there also arises the problem that a dispersion is caused in the etched amount due to the difference in etching rates on the Al electrode pad formed by sputtering. Due to these problems, a surface oxide film 23 is left locally on an Al electrode pad 22 as shown in FIG. 4(b). As a result, the substituting reaction between Al and Zn, in the pre-treatment, for depositing an electroless Ni or Ni alloy plating comes not to be performed with uniformity and denseness, as shown in FIG. 4(c). Accordingly, a dispersion takes place in the deposition of a Zn film 25.
Next, an electroless Ni or Ni alloy plating treatment is carried out. This plating treatment comprises the fact that Ni or Ni alloy which becomes nuclei is formed by the substituting reaction thereof with Zn, and then, Ni or Ni alloy is self-deposited with the nucleating Ni or Ni alloy as nuclei. However, since the deposition of the Zn film 25 disperses, the deposition of the nucleating Ni or Ni alloy also become coarse, producing a dispersion. Thus, a projected electrode 26 of Ni or Ni alloy which has projections and depressions on its surface is formed as shown in FIG. 4(d). Further, due to the self-deposition of the Ni or Ni alloy, the projected electrode is increased in its film thickness, but, since the nucleating Ni or Ni alloy is coarse, its grain size also becomes large, so that gaps are produced among the grain boundaries. Due to the existence of these gaps, corrosion is caused by the plating solution or impurities penetrate in at the subsequent plating step, thus lowering the reliability.
Further, since the Ni or Ni alloy portion becoming a core is coarse, the adhesion between the Al electrode pad and the projected electrode is also weak. FIG. 4(a) indicates the state in which the surface oxide film exists on the Al electrode pad.
Further, as disclosed in Japanese Unexamined Patent Publication No. HEI 9(1997)-306871, in case an Ni plating is directly made onto the Al electrode pad to form the projected electrode after the surface oxide film on the Al electrode pad is removed by sputtering, it is necessary to perfectly prevent the re-formation of a surface oxide film on the Al electrode pad. For this purpose, after the surface oxide film on the Al electrode pad is removed by sputtering, the sample must be moved directly into an inert atmosphere to continuously perform a plating step. This means that the provision of a large-scale apparatus becomes necessary.